Cerebellar Units Show Several Types of Early Responses to Telencephalic Stimulation in Catfish

Abstract

The responses of cerebellar units following electrical stimulation applied to the area dorsalis centralis (Dc) of the telencephalon were studied in the siluroid teleost Ictalurus nebulosus. Two kinds of units are distinguished on physiological criteria, identified as Purkinje and eurydendroid cells (the efferent neurons of the cerebellum in teleosts equivalent to cells of the deep nuclei in other vertebrates). A high proportion of both kinds of units in the corpus cerebelli are sensitive to such stimulation. Each kind of unit shows several consistent response types. Purkinje cells fire simple spikes spontaneously at the rate of 8–50 spikes/s and respond to a single shock to Dc with an initial latency of 34–64 ms. The response can be one of the following types: (1) inhibition alone, with a duration of 0.3–1.5 s; (2) initial inhibition for 0.04–0.2 s, followed by postinhibitory rebound, or (3) initial excitation followed by inhibition which may or may not be followed by a late excitation. It is suggested that the initial excitation and initial inhibition reflect the activation of mossy fiber-granule cell-Purkinje cell circuitry and mossy fiber-granule cell-inhibitory interneuron-Purkinje cell circuitry, respectively. Indirect evidence suggests the involvement of climbing fibers, but their characteristic complex spikes are rarely seen. Changing the stimulation sites within Dc does not appear to change the response pattern but may alter the threshold intensities, latencies and amplitudes. Changing stimulation frequency has complex effects depending on the response type. Purkinje cells responding with initial excitation are located along the lateral edges and along the midline of the corpus cerebelli; units responding with initial inhibition are more often found in an intermediate zone. This suggests three sagittal bands on each side. The contralateral cerebellum has a relative excess of Purkinje cells with initially inhibitory response. Putative eurydendroid cells show either initial excitation or a pattern of inhibition, excitation, inhibition. The initial latency is longer than in Purkinje cells. The contralateral cerebellum has a relative excess of eurydendroid cells with initially excitatory responseThe high proportion of units that respond to Dc stimulation, their complex dynamics, diverse response types and compartmentalization point to the importance of the cerebrocerebellar influence in teleosts. Principal differences from mammals are the much longer latencies and less differentiation according to place stimulation in the cerebrum.

Cerebellar Units Show Several Types of Long-Lasting Posttetanic Responses to Telencephalic Stimulation in Catfish

“…As reported earlier [Lee and Bullock, 1990], two kinds of large unit spikes can be distinguished. Attri buted to Purkinje cells are rare complex spikes and common, brief, ongoing (6-60/s) simple spikes.…”

“…Its output provides a significant indirect influence on the cerebellum, and this has been thought, without direct proof, to involve both mossy and climbing fibers [Lee, 1984;Lee and Bullock, 1990],…”

“…One third of these units be longed to the type that responds to single pulse stimu lation with an early excitation followed by inhibition. The other two thirds responded with an early inhibi tion followed by strong postinhibitory rebound [Lee and Bullock, 1990]. As a group, these 15 units had a mean spontaneous frequency of 16.9 ± 6.3 spikes/s (mean ± SD), ranging from 6 to 27.1 spikes/s before the application of the pulse train.…”

“…Repetitive stimulation for 16-32 stimuli at the rate of 2-4 Hz reduces or enhances the Purkinje cell's response to a single shock as a test of excitability. It does not alter the pattern of the re sponse; cells that respond with early excitation con tinue to respond with early excitation, although the amplitude of the response, assessed by the height of poststimulus histograms, might be increased or de creased [Lee and Bullock, 1990].…”

“…During a study of the cerebellar responses to tel encephalic stimulation in a teleost [Lee, 1984;Lee and Bullock, 1990], it was discovered that long-lasting in creases or decreases in the mean frequency and pat tern of ongoing spike discharges of Purkinje cells fol lowed stimulus trains of a few seconds. The purpose of these experiments is to explore and quantify these effects, which fall into the time domain often called posttetanic potentiation or plasticity [Teyler and Discenna, 1984].…”